1. Field of the Invention
The present invention relates to a structure for mounting a semiconductor device to a mounting substrate, such as a printed circuit board, and a semiconductor device itself.
2. Description of the Related Art
A basic concept of a prior art structure for mounting a semiconductor device to a printed circuit board will be described below with reference to FIG. 4.
First, a general structure of a semiconductor device 10 and a printed circuit board 12 will be explained.
An electrode 14 is formed to be exposed on a surface of the semiconductor device 10.
A metallic bump 16 is attached in advance on a surface of the electrode 14. The metallic bump 16 is made of eutectic solder to have a generally spherical or columnar shape.
A pad 18 is formed on a surface area of the printed circuit board 12 to which the semiconductor device 10 is mounted at a position corresponding to that of the electrode 14 formed on the semiconductor device 10.
Next, the structure for mounting the semiconductor device 10 (onto the printed circuit board 12) will be explained.
The semiconductor device 10 is placed on the mounting area of the printed circuit board 12 so that the respective metallic bumps 16 attached to the respective electrodes 14 of the semiconductor device 10 are located on the corresponding pads 18 of the printed circuit board 12.
While maintaining this positional relationship, heat is applied to melt the metallic bumps 16 of eutectic solder.
Thus, the semiconductor device 10 is mounted onto the printed circuit board 12.
In the above-mentioned prior art structure for mounting the semiconductor device, the spherical or columnar metallic bumps 16 of eutectic solder are molten to flow over the entirety of the pads 12 on the printed circuit board 12 side. Thereby, the metallic bump 16 collapses in shape as shown in FIG. 4 to reduce the height H.
Particularly, when a semiconductor device is small in size, such as indicated at the reference numeral 10 shown in FIG. 5 wherein an insulating protective film 30 is formed on a passivation film 28 through which electrode terminals 26 of aluminum are exposed and circuit patterns 32 electrically connected to the electrode terminals 26 are formed on the protective film 30 and wherein electrodes 14 (of a columnar shape in FIG. 5) are formed on the circuit patterns 32 which are embedded in a mold resin 36 to solely expose tip ends of the respective electrodes 14 to which metallic bumps 16 are attached, or one, as shown in FIG. 6 wherein metallic bumps 16 are directly attached to electrodes 14 formed on an active device surface of a semiconductor chip 24, it is necessary to minimize a diameter of the metallic bump 16 because the arrangement pitch of the electrodes 14 is small. As an example, when a spherical metallic bump 16 of approximately 0.45 mm diameter is formed, a height thereof after being molten will be in a range from approximately 0.3 to 0.32 mm.
However, it has been found that, in a connection-reliability estimation test wherein the semiconductor device 10 is subjected to a heat shock while being mounted to the printed circuit board 12 there is a tendency in that, the higher the height of the metallic bump 16 after being mounted to the printed circuit board 12, the better the test result. Accordingly, it is desirable to maintain the height of the metallic bump 16 as large as possible to realize the high reliability.
Also, when the semiconductor device is subjected to heat shock, fatigue (for example, cracking) may occur in the vicinity of an end of the metallic bump 16 on the semiconductor device 10 side or an end thereof on the printed circuit board 12 side to cause a failure of connection. Accordingly, it is favorable to increase the strength of the end region having a low strength to realize high reliability.
Particularly, since the original height of the metallic bump 16 is low in the semiconductor device 10 shown in FIG. 5 or 6, it is most important to maintain the original height as much as possible to facilitate the connection reliability.